To improve reliability on joining sections by tightly unifying the irregular sections of an inner ring, the irregular sections of the joining sections of upper and lower piles and an outer ring.

An outer ring 11 is fitted movably to a lower pile 1, and each end face of the end disk 2 of the lower pile 1 and the end disk 4 of an upper pile 3 is abutted and joined. The projecting strips 8a, 8b of the inner walls of inner rings 7a, 7b are installed so as to be inserted into ring-shaped grooves 5, 6 formed to the lower pile 1 and the outer wall of a cylindrical section 2a fitted and fixed onto the upper pile 3. The outer ring 11 is put to the outsides of the inner rings 7a, 7b. Taper pins 14 are pressed and inserted into tapered holes 13, and the ring-shaped grooves of the end disks 2, 4 and the projecting strips 8a, 8b of the inner rings 7a, 7b are fitted tightly while the inner rings 7a, 7b and the outer ring 11 are unified closely.